Jordan Spieth plays out of the rough on the sixth hole during a practice day at Royal Troon Golf Club in Scotland on Tuesday. Picture by PA

JORDAN SPIETH has labelled his decision to withdraw from the Olympics as the hardest of his life and feels it will "loom" over him throughout the Games.

But the two-time major winner refused to elaborate on the "health concerns" which led to him pulling out of the American team at the last minute, even after sitting next to Rickie Fowler as he confirmed his own participation on social media.

Asked to specify what medical advice he had received, Spieth said: "No, that's personal. I can't. I can tell you that I'm not specifically pinpointing any one thing in my health concerns either."

More than 20 male players have now opted out of competing in Rio, with Spieth's fellow members of the world's top four - Jason Day, Dustin Johnson and Rory McIlroy - all citing concerns over Zika, a mosquito-borne virus which has been linked to defects in newborn babies and Guillain-Barre, a rare neurological syndrome which causes temporary paralysis in adults.

"I didn't cite that, so please don't do that for me," added Spieth.

"That [Zika] is not the only one."

Spieth had previously cited "other bacteria stuff" and "security threats" in Brazil as factors in making what he called "probably the hardest decision I've ever had to make in my life". The 22-year-old added: "This was harder than trying to decide what university to go to. Whether to turn professional and leave school. This was something I very much struggled with, I bounced back and forth with and, ultimately, a decision had to be made yesterday and so I made it.

"I will continue to carry it with me through these Games and for a while I think. It will loom over me throughout the Olympic games, for sure. I'm sure at times I'll be pretty upset that I'm not down there. I'm a huge believer in Olympic golf and hope to play in four or five in the future if I have the opportunity. This year I just had to try and weigh a risk that doesn't present itself every year.

"Not everybody's going to understand. I can understand why people are sceptical. They're as passionate about the Olympics as I am. They also are not in my shoes. I feel that many, if not all of you, would have made the same decision I made if you were in my shoes. Whether you believe it or not, so be it. Do I think it looks bad on golf? Maybe. Again, I'm making the decision of what I think is best for me. I don't feel like I have to carry the torch for the sport or anyone else."

Spieth, who arrived at the Open last year seeking the third leg of an unprecedented calendar grand slam after winning the Masters and US Open, said he will not defend his John Deere Classic title in the same week as the Olympics, feeling it would be "inappropriate". And he also conceded that there could have been an over-reaction to the threat of Zika from male golfers, with South Africa's Lee-Anne Pace the only female player to pull out to date.

Former US Open champion Justin Rose, who will represent Great Britain along with Masters champion Danny Willett, hopes that Zika does turn into a "non-event" but added: "At the same time, no-one can stand there and categorically tell you you're going to be okay, and that's the problem.

"Obviously, with golf being an outdoor sport played around the water out there, five, six hours out on the golf course, seven days a week, you are probably at a higher risk than most other athletes in most other sports who are in much more of a contained environment.

"I do sympathise with the guys who are at that phase of their lives where either their wives are pregnant or having children in the future. It's been sort of a sad situation to see those sorts of guys pulling out. But you've got to respect their position. You do have to put your health and family first from that point of view. The golfers are skipping Rio, not the Olympics."

Rose will have just turned 40 by the time of the 2020 Olympics in Tokyo and added: "I am treating it as a once-in-a-lifetime opportunity: "At this stage in my career, when it's once every four years, I think it's something you can certainly make an exception for, and that's been my attitude towards it.

"Just being a part of Team GB, in a sense you feel like you're part of something bigger than just your individual sport as well. I've never been to an Olympic Games in any capacity. To go, obviously, as an athlete is a huge honour, but I want to take in the Games as a whole."